Re: [PATCH] ubi: check kthread_should_stop() after the setting of task state

From: Zhihao Cheng
Date: Tue Aug 04 2020 - 22:23:32 EST

在 2020/8/5 5:56, Richard Weinberger 写道:
On Tue, Aug 4, 2020 at 4:58 AM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
Oh, you're thinking about influence by schedule(), I get it. But I think
it still works. Because the ubi_thread is still on runqueue, it will be
scheduled to execute later anyway.
It will not get woken. This is the problem.

op state of
ubi_thread on runqueue
if (kthread_should_stop()) // not satisfy
ttwu_do_wakeup TASK_RUNNING Yes

// prev->state is TASK_RUNNING, so we cannot move it from runqueue by
deactivate_task(). So just pick next task to execute, ubi_thread is
still on runqueue and will be scheduled to execute later.
It will be in state TASK_RUNNING only if your check is reached.

If kthread_stop() is called *before* your code:
+ if (kthread_should_stop()) {
+ set_current_state(TASK_RUNNING);
+ break;
+ }

...everything is fine.
But there is still a race window between your if
(kthread_should_stop()) and schedule() in the next line.
So if kthread_stop() is called right *after* the if and *before*
schedule(), the task state is still TASK_INTERRUPTIBLE
--> schedule() will not return unless the task is explicitly woken,
which does not happen.
Er, I can't get the point. I can list two possible situations, did I miss other situations?

  if (kthread_should_stop()) {
  schedule()                            -> don't *remove* task from runqueue if *TASK_RUNNING*, removing operation is protected by rq_lock

  set_bit(KTHREAD_SHOULD_STOP, &kthread->flags)
  wake_up_process(k)             -> enqueue task & set *TASK_RUNNING*, these two operations are protected by rq_lock

Situation 1:
P1_set_current_state               on-rq, TASK_RUNNING -> TASK_INTERRUPTIBLE
P1_kthread_should_stop        on-rq, TASK_INTERRUPTIBLE
P2_set_bit                               on-rq, TASK_INTERRUPTIBLE , KTHREAD_SHOULD_STOP
P2_wake_up_process             on-rq, TASK_INTERRUPTIBLE -> TASK_RUNNING , KTHREAD_SHOULD_STOP
P1_schedule                           on-rq, TASK_RUNNING , KTHREAD_SHOULD_STOP
P2_wait_for_completion        // wait for P1 exit

Situation 2:
P1_set_current_state             on-rq, TASK_RUNNING -> TASK_INTERRUPTIBLE
P1_kthread_should_stop       on-rq, TASK_INTERRUPTIBLE
P2_set_bit                             on-rq, TASK_INTERRUPTIBLE , KTHREAD_SHOULD_STOP
P1_schedule                          off-rq, TASK_INTERRUPTIBLE , KTHREAD_SHOULD_STOP
P2_wake_up_process             on-rq, TASK_INTERRUPTIBLE -> TASK_RUNNING , KTHREAD_SHOULD_STOP
P2_wait_for_completion       // wait for P1 exit
Before your patch, the race window was much larger, I fully agree, but
your patch does not cure the problem
it just makes it harder to hit.

And using mdelay() to verify such a thing is also tricky because
mdelay() will influence the task state.